The present invention provides an earthing switch for a metal-clad, high-voltage switchgear insulated with compressed gas having a mobile earthing contact, which is arranged so as to be insulated from the earthed cladding, and provides an electrical connection that bypasses the insulation arrangement of the earthing contact to prevent pre-arcing and voltage surges.
Earthing switches of this kind, as disclosed, for example, by the European Patent Document No. EP-A 0 075 922, are used not only to transfer the earth potential to the conductor of the switchgear carrying the high voltage, but also for measuring purposes during normal operation, in particular when electric energy is not being transferred. The electrical connection that bypasses the insulation arrangement of the earthing contact must be open in order to conduct measurements.
The earthing switch is jeopardized by pre-arcing to the mobile earthing contact in the area of the insulation arrangement when the earthing switch is moved to the closed position. In the course of the closing movement, charges of considerable voltages can still be present on the conductor of the assembly, which could abruptly lead to an arc-over shortly before the earthing contact comes in galvanic contact with the conductor. Such a sparkover causes transient surges, which can overload the insulation arrangement, which is typically rated for voltages of only a few kV.
U.S. Pat. No. 4,413,166 discloses a disconnect switch, whereby one of the pair of electrodes that is separated from the other is forced to make galvanic contact with the earth potential when it is in the open position.
European Patent Document No. EP-A-0 100 510 discloses an earthing switch that establishes an earthing contact by way of a resistor. No provision is made therein for a temporary direct grounding of one electrode of the earthing switch to protect from transient surges.